Best solution is to just login to the web interface for that provider and reset the password.
But if you don’t need access to that account, either remove it from eM Client, or disable the IMAP service. To disable the service go to Menu > Accounts and click on the General tab for that account. Scroll down to Services and untick IMAP. This will hide the IMAP folders in eM Client, and will no longer accept push notifications from the server, so no password prompt. To disable sending on that account, you can also untick the SMTP service.
If it is an Exchange account, untick Exchange Web Services.